Open end spinning machines, such as are known, for example, from the manual "Autocore" of W. Schlafhorst & Co., have a plurality of aligned work stations, which are serviced during the spinning operation by an automatically operating service unit, often referred to as a piecing cart, which can be moved along the work stations.
As described, for example, in German Patent Publication DE 43 13 523 A1, the piecing cart can exchange finished yarn cheeses for empty winding tubes and can also repair yarn breaks at the spinning stations. In case of a yarn break, the piecing cart first cleans the respective spinning station and thereafter reattaches the yarn with the fibers being spun within the spinning station by a so-called piecing operation.
Automatic yarn piecing by the piecing cart takes place within a predetermined range of rotational speeds (rpm) of the spinning rotor which are optimal for piecing. Accordingly, the piecing cart is equipped with a device for measuring the rotor speed. Following cleaning, the cart starts the speed measurement when a rotor brake is disengaged from the rotor shaft and the rotor is accelerated to its operating speed. The actual yarn piecing cycle is started at predetermined optimal piecing speed.
So that the pieced yarn has a continuous thickness and twist during the continuing acceleration of the rotor to its operating speed, the draw-in of fibers into the rotor and the draw-off of yarn from the rotor must be accelerated in the same way as the rotor speed increases.
During the piecing process the piecing cart therefore takes over the drawing-in of the fiber into the spinning unit as well as the yarn draw-off from the spinning unit, and at the same time also accelerates a drive of the cheese onto which the yarn is being wound. The drive of the cheese, which may also be called a winding bobbin, is performed in this case by means of a so-called lap drive. The lap drive has a drive roller disposed at the end of a drive arm, which is pivoted against the surface of the winding bobbin for accelerating it.
This known method has proven effective in actual use and is employed in connection with a number of open end rotor spinning machines. However, as the productivity of open end rotor spinning machines has continued to increase, very high rotor speeds and correspondingly high yarn draw-off speeds have become common. This increase in the yarn draw-off speed makes it likely that in the future difficulties will arise during the piecing process, since the present-day lap drives are not well suited to accelerate large yarn bobbins, which have a correspondingly high moment of inertia, sufficiently that the yarn winding speed can follow the high acceleration of the yarn draw-off device. In this context it must be taken into consideration that the moment transfer by means of a friction wheel is limited, even if the acceleration capability of such drives can be optimized, for example by means of the material selected for the drive roller, its contact pressure against the winding bobbin, its looping around the yarn roller, the pressure surface of the latter and its profile.